


A Novel Experience

by TheRechercheRambler



Series: A Chance Meeting [1]
Category: Undertale (Video Game)
Genre: Alternate Universe - Underfell, Gen, Pre-Undertale, Undertale/Underfell crossover, also pre-Underfell
Language: English
Status: Completed
Published: 2015-11-26
Updated: 2015-12-27
Packaged: 2018-05-03 12:36:29
Rating: Teen And Up Audiences
Warnings: No Archive Warnings Apply
Chapters: 11
Words: 17,310
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/5291093
Author URL: https://archiveofourown.org/users/TheRechercheRambler/pseuds/TheRechercheRambler
Summary: <blockquote class="userstuff">
              <p>What began as yet another loop takes a turn for the strange in the days before the human is due to arrive, and an aberration in the fabric of space leads to a few unexpected chance meetings.</p>
            </blockquote>





	1. Chapter 1

**Notes for the Chapter:**

> Just a head's up, this begins with the Underfell guys, not the Undertale ones.

“SANS, YOU INSUFFERABLE LAZY BONES! GET BACK TO YOUR POST THIS INSTANT!”

  
Sans groaned, opened his eyes, and looked around. Wha-? Where..? Oh, right. The forest. He'd gone off to take a nap by the door to the Ruins, as he always did, though this nap somehow never failed to disorient him. And now that Papyrus had found him, his brother was angry.

“hey, bro.” Sans said sleepily.

More to the point, Papyrus was on the verge of a tantrum. The taller skeleton was stamping his boots in the snow, fists clenched and eyes narrowed. When he'd gotten that out of his system, he went back to yelling at his brother.

“DID YOU… DID YOU LEAVE YOUR POST TO TAKE A _NAP_?! I'VE HAD IT WITH YOUR LAZINESS, SANS!” he shouted, shaking his fist as Sans got to his feet. “IF A HUMAN HAD PASSED BY HERE...”

Sans glanced left and right. The only footprints in sight were Papyrus's, and no snow had fallen on him while he'd been napping. And if Papyrus had just arrived at this spot to yell at him with this spiel, that pesky kid wasn't due for another day at least. He had long ago stopped being curious as to whether it would be one of the times that the dumb kid would try to kill everyone, or befriend everyone (which was always the less successful venture).

“well, nothing happened.” Sans said tiredly. “get stuffed.” he added, as nonchalantly as he dared.

Papyrus fumed for a moment, grinding his sharp teeth, right orbital twitching. “YOU USELESS LAYABOUT!” he yelled suddenly.

“jerk.”

“PATHETIC SLUGGARD!”

“b-” Some part of Sans was tempted to say _bonehead_ , as he always was, but… Papyrus had always had a low tolerance for puns, which had just worsened over the years. Provoking him was the last thing Sans wanted to do. He quickly settled on “brute” instead, although that was practically a compliment in the Underground.

These days, they exchanged insults almost every time they saw each other, although Sans could never figure out why Papyrus let him get away with it. They weren't exactly in top form at the moment, but it was the thought that counted, in some warped way.

After the briefest of pauses, Papyrus decided to let Sans have it again.

“YOU CAN'T KEEP DOING THIS, SANS! SHIRKING YOUR JOB AND- AND NAPPING! AND YOUR TRAPS, SANS! HAVE YOU EVEN SO MUCH AS CHECKED ON YOUR TRAPS TODAY? I'M NOT GOING TO DO YOUR JOB FOR YOU! AND IF UNDYNE FINDS OUT THAT YOU'VE BEEN SLACKING- OR IF YOUR LOAFING JEOPARDIZES MY CHANCES OF JOINING THE ROYAL GUARD-” the taller skeleton cut off, looming over Sans menacingly and giving his brother a furious glare.

Silence was allowed to reign for another quick moment, as Papyrus stared down at Sans, letting the implied threats sink in. Then, suddenly, Papyrus lunged at Sans- who flinched- grabbed him by the arm, and roughly hauled him to his feet.

Beads of sweat began to form on Sans's forehead. Hopefully Papyrus wasn't going to start laying into him magically or physically as well as verbally. He was fairly certain that he wouldn't- he'd never gone that far- Papyrus was well aware he only had one hit point!- but in spite of how many times he'd looped through situations like this, Sans could never shake the anxiety that cropped up whenever they got into an argument.

“NOW STOP MALINGERING, AND GO TAKE CARE OF YOUR TRAPS AND GET BACK TO YOUR POST, YOU WORTHLESS GOOD-FOR-NOTHING!” Papyrus shouted, shoving Sans away and releasing him in the same motion.

Sans turned, and began to trudge back to his post, hoping his nerves didn't show. The taller skeleton huffed and watched him leave; his brother knew he was about to let off one final parting shot.

“DON'T LET IT HAPPEN AGAIN! IF I HAVE TO TELL YOU ONE MORE TIME-”

-and then, suddenly, Papyrus's tirade was cut off. Sans hesitantly looked back, startled; Papyrus had never stopped in the middle of a lecture like that before. His orbitals went wide with shock.

Papyrus was gone. The only signs that he had ever been there at all were his bootprints in the snow.

“papyrus?” Sans called, looking around nervously. “heh… bro? where'd you go?”

He got no response, and decided to try another tactic, more out of desperation than out of any hope that it would work.

“...jerk? dastard? pathetic excuse for a guardsman?” That last one, at least, would have usually brought Papyrus running if the taller skeleton had been within a one-mile radius of whoever was saying it.

Dread colder than the weather of Snowdin grew within him, feeling for all the world like solid, heavy lead weighing him down. This was not good. Whatever had happened to Papyrus could easily happen to him, too; it could happen in the next minute, the next second-

For a second, nothing happened. Then, for the next second, and the one after it, nothing continued to happen. Sans was entirely alone.  
With no sign of immediate danger in sight, sorrow suddenly reared its head and began to creep in alongside the dread. Papyrus was gone. His little brother was gone. The only family he had was gone. The closest thing he had to a friend- it had been a long time since he'd come that close to describing anyone with that word- was gone. Papyrus was gone, and in spite of how terrible their relationship had been recently, Sans felt horribly, utterly sad and alone.

This was not supposed to happen. Sans had seen Papyrus be- he had lost Papyrus before, many times in fact, and no matter how many times it cane undone it still hurt, even though it had never stuck, but it had always been at the hands of that stupid kid, and always much later in the day, after a pitched battle, not in the forest of Snowdin and not out of the blue like this. Somehow, this was worse; it was an entirely new anomaly, and its sheer novelty alone was frightening.

He slumped back down against the tree, staring ahead in shock, and then he happened to notice something in front of him. In spite of his sloth he had always been very observant, and now he saw that, among the large, pointy bootprints that Papyrus had only seconds ago stamped into the snow, was a smaller, rounder set of prints.

Now it was anger's turn to shine, as Sans got up to take a closer look at the strange prints- if a stranger had been there, then it must have been deliberate, and Sans wanted to know who had done it. The prints were about as deep as Papyrus's prints, so they must have come from someone who weighed a bit. Oddly, didn't look like they'd come from boots; almost everyone in the Underground, it seemed, wore boots those days. Sans put his own boot beside the new print, and saw that they were almost the exact same size and depth. The main difference was the shape, and that at the toe end, the new prints looked somewhat indistinct, almost... fuzzy.

Had those come from _slippers_?

A branch cracked behind him. Sans whirled around, fearful again, but saw nothing. Well, Papyrus obviously wasn't there, and there were no other clues, so Sans decided it was time to get the heck out of Dodge and take a short cut back home.

This he accomplished the usual matter of seconds, but something felt… off. Slipping between cracks in space and time never felt quite right, granted, but it was usually better than this. The space around him seemed to be reverberating, like a shockwave had just passed through it.

A thought struck him. Perhaps it was time to go look at what was in the workshop. He hadn't so much as set foot in the room in ages, only once or twice since the accident, and only then to try and get better at taking shortcuts, but if he could wrap his head around the rest of the research he might be able to figure out just what the heck had happened.

 

Back in the forest the red- and black-clad skeleton had just left, someone else was trying to make sense of the situation from the cover of the dark conifers. Now alone, he cautiously trod back into the open, and gave the area a quick look-around himself. When his own survey was finished, he looked down the path to Snowdin Village, and vanished into thin air.

**Notes for the Chapter:**

> My first fic posted on an actual fic archive instead of just Tumblr! Hope you like it. I've got five chapters already written, and several more outlined, so this should keep going on for a bit. I will be trying to update every three days, so Chapter 2 will be coming on Sunday.


	2. Chapter 2

**Summary for the Chapter:**

> While Sans tries to make sense of the situation, Sans tries to make sense of the situation.

Not long ago, but in a universe fairly far away…

Sans had been napping at the door to the Ruins, as he always did at that point in the loop. Toriel- well, the woman behind the door, if anyone else had asked him at that time- hadn't been there; she would be busy with taking care of Frisk at that point. Hopefully he wouldn't have to give the kid a bad time in this loop.

He had woken up a little earlier than he usually did, however; that was one of the few things that varied from timeline to timeline, as it was connected to his emotional state almost as much as it was to how tired he was actually feeling. When he awoke he was feeling a tad hungry (as usual), so he decided he'd head to Grillby's. The short skeleton stood up and took a few steps away from the door before taking a shortcut.

Immediately, he could tell that something had gone terribly wrong. As soon as he left normal space, he felt as if something had just hit him, hard. Space seemed to surge around him, rushing outwards like the shockwave of an explosion. Sans thought better of going to Grillby's and went back to normal space, exactly where he'd been.

Well, that was where he had _expected_ to end up. It didn't go quite like that in reality; he was back in Snowdin Forest, to be sure, but he was no longer alone. Someone else- another skeleton, to Sans's bewilderment- was trudging away down the path ahead of him.

They looked an awful lot like him, except they were wearing a lot more black.

Sans was shocked enough by this turn of events that he took another shortcut, more out of instinct than anything else. He did not want the strange monster to see him until he had some idea of what he was dealing with. The shortcut was kept very short, just in case, and in an instant he was watching the path from behind the dark trees of the forest.

Just in time, too; the other monster, who looked even more like he did from this angle, had somehow noticed his arrival. Sans didn't catch all that the strangely Sans-like skeleton was saying, but he did make out a name: Papyrus.

His dark-clad doppelgänger poked around where Sans had briefly stood, and then disappeared. Apparently they were alike in more than just looks. Sans left his cover to go see what the other him had been looking at. Then, seeing as the stranger hadn't reappeared with a look of surprise on his face, Sans figured it was probably safe to take a longer shortcut.

Things were still… not quite normal, what with space rippling around him, but it felt less turbulent than it had when he'd last tried it. He was hoping to find himself behind his and Papyrus's home, and when he emerged in normal space he did see a building before him that was the same shape and size, with a shed beside it. There were no strings of lights under the eaves of the house, and all of the windows that he could see were dark and grimy. The weather of Snowdin usually had no effect on Sans, what with him being a skeleton and all, but as he looked up at the house he began to feel cold.

The other houses nearby looked equally unwelcoming. Wherever this was, it was not the Snowdin that Sans knew.

Well. There wasn't much more to be learned from standing about in the snow. Sans's eyes drifted up to look at the window that, in his Snowdin, looked out from his own room. It appeared to be the dustiest window of them all.

Scarcely ten seconds later, and Sans was squinting about a room that was a dark parody of his own, in terms of both light level and décor. A shabby mattress with a few broken springs sticking out of it had been half-heartedly shoved against a wall. There was a coverless pillow gathering dust on one end of it, and black sheets had been not so much made as draped over the whole sad affair, and doing a terrible job of hiding the springs poking up against them. Sans glanced at the corner; no trash tornado, only some gathering storm clouds of dirt and grime. In fact, there was a strange lack of debris littering the floor.

In the gloom, the only other objects Sans could see besides the bed were a black drawer, a floor lamp with a black lamp cover- the profusion of black was a touch confusing- a grimy trombone leaning haphazardly against it that clearly hadn't been so much as touched in years, and a treadmill, rather inconveniently placed in the middle of the room, that the occupant hadn't bothered to move. The middle of the floor was less snugly blanketed in dust than the corners were, and a cursory inspection revealed that the accumulated dirt had been swept- shoved- somehow moved- into the space underneath the drawer, which was now home to a thriving warren of dust bunnies.

Had Sans known more of surface pop culture, he might have compared the experience to stumbling into the Twilight Zone. But he hadn't, so he didn't.

If he pretended that it wasn't a practical impossibility, and didn't just dismiss it as a thoroughly silly idea besides, Sans saw that it was obvious that somehow, he had found himself in an alternate version of the Underground.

Staying in the murky not-quite-his room was rapidly becoming extremely disconcerting, and so it was partly because of this, and partly because in such a place it was probably safer to keep moving, Sans headed over to the door.

The second-story hallway was much less dusty, he saw as he quietly closed the door behind him, but with the lights out it was no less gloomy. There was a framed picture of a femur on a red background on the wall. After a brief moment of hesitation, Sans went and looked down over the banister to see what the rest of this house- for in his opinion, it could hardly be called a home- was like.

There were no post-it notes or sprinkles or books in sight. On one side of the room was a plain black metal table with some neatly-stacked papers on it, weighted down by a rock. A similarly plain and metal stand was over on the other side of the room, with nothing on it. Beside it was a trio of small, gray, thinly-padded and armless chairs. In spite of being unoccupied, the room managed to radiate a thoroughly unwelcoming feeling.

The big-boned skeleton was about to move on again, perhaps to see the room corresponding to Papyrus's, and see if he could get more evidence for the alternate universe theory, when he heard a very faint, brief noise behind him. This was not an easily described noise; while probably best put into onomatopoeia as _fwish_ with a bit of _scritch_ thrown in, it was really somewhere between a fragment of noise chopped out of the sound of a running suction hose, and nails scratching at the fabric of space. This peculiar, unnatural noise was very hard to hear at a distance, unless one knew what what they were listening for, and it was one Sans was very much familiar with.

It was the sound his shortcuts and other space shenanigans made. It came from the room he had just left.

He silently backed down the hallway, keeping his orbitals glued to the door to not-quite-his room. Nothing happened for a moment, and when he was at the door to maybe-his-brother's room, he disappeared into thin air.

 

Down in the old workshop beneath the foundations of the house, the Sans native to that universe had been digging through old papers, papers he'd scarcely glanced at for years. He went through disorganized cabinets and drawers, full of reports and essays and photographs and files, and a single dulled badge. He was haphazardly sorting the papers into a few scattered categories. Papers about souls were hurriedly set in a sagging almost-pile; schematics and plans for the Core, in a handwriting that was almost, but not quite, entirely unlike anything legible were brushed aside in a heap; lab reports on experiments for enhancing a monster's powers- these Sans was sorely tempted to crumple up and destroy, but old fears and habits prevented him from doing so, and he threw them across the room instead.

The smallest and neatest group of papers Sans kept close to hand; it contained all information pertaining to space and its manipulation. He had already found all of the papers he had examined back when he had still been trying to get a handle on taking shortcuts, but he knew there was more, lurking somewhere in the shadowy places of the filing cabinets. It didn't take him much longer to hound them out. Clutching the small stack of papers, Sans left the room, with its hollowed-out drawers and scattered papery mess, behind.

He went to his own room, for lack of a better place to go, and when he chanced to look down at the floor the papers fell from his hands. Someone had been in his room- he could see the faintest shapes of small footprints in the dust. Fuzzy footprints. _Slippered_ footprints.

“what the frick?!”

 

**Notes for the Chapter:**

> Writing about the Underfell house was a tad tricky, trying to strike a balance between keeping it close enough to the original to be recognizable, and making it different enough to not just be cut-and-paste with a different color scheme.
> 
> In case anyone's wondering, when the crossover initially happened, Underfell!Sans didn't hear the sound of Undertale!Sans taking a shortcut because it was drowned out by Underfell!Papyrus's shouting.
> 
> Unfortunately, due to college stuff, I'll have to postpone posting Chapter 3 until tomorrow.


	3. Chapter 3

**Summary for the Chapter:**

> Papyrus meets Papyrus; confusion all around.

**Notes for the Chapter:**

> This one begins with Underfell Papyrus, continuing where he left off in Chapter 1.

“-THEN! I'LL!!!”

When the unexpected unpleasantness had ended, Papyrus had continued on with his tirade, mostly because doing something normal, like coming up with a threat (even one both he and Sans knew to be empty) was a good way to avoid thinking about the mind-bendingly unthinkable. Or, worse yet, showing fear, an idea that was almost equally awful. Without anyone around to threaten, however, that was an exercise in futility; his mouth was still open, patiently waiting for his thoughts to start stringing themselves together again and forming into words.

Finally, he settled on an angry, “SAAANS!” in the hopes that maybe, just maybe, this was somehow his brother's fault; if that was so, then there was someone he could blame, someone from whom he could demand an explanation, and, perhaps, punish for it so that it would never happen again. Undyne loved finding people to blame and punish- she especially loved that second part- and Papyrus knew he had to emulate the best if he wanted to get into the Royal Guard. He was a quick learner. Although, he didn't think he could _actually_ hurt Sans (but he wouldn't be caught dead admitting to such shameful weakness). Everyone- especially Undyne- would jump at the chance to exploit it, while Sans already knew, and would just use it to get away with more slacking.

Well. That was the main reason he called for his brother; yes, definitely the _main_ one. But, Papyrus had just noticed some worrisome things. The door to the Ruins was purple, instead of its usual red; that his footprints had suddenly been erased; that the distant glow of Snowdin Town was a warm yellow, rather than a harsh orange; and all things considered, Papyrus rationalized to himself, he would prefer to have his brother nearby- where he could keep an eye on him!- in such strange circumstances.

After all, Sans only had one hit point, and no defense or attack to speak of (something that neither brother had ever told anyone else- what Undyne didn't know wouldn't hurt her, or them). It was a miracle that he had lasted as long as he had, and now he was missing… Papyrus didn't want all of his hard work, looking out for Sans and pushing him to become stronger, to go to waste. That was all.

“SANS! COME OUT HERE, NOW!” he roared at the emptiness of the forest, hoping that he might scare his brother into the open. Sans was a terrible coward, but he'd never run from Papyrus before. But nobody came.

Papyrus had for some time suspected that his brother could, in some way, cajole space into making him be where he wanted to be. He had a curious knack for suddenly being right behind you when you turned around, and then being back in front of you when you checked to see what had happened. That wasn't a knack he had used much for a few years- though Papyrus couldn't quite figure why- but he still tended to show up in unexpected places. If he could make himself move in space, perhaps Sans had somehow done something to Papyrus? If he had, then hopefully he could undo it. If only he would just come back...

“SANS!!!” he screamed, voice higher than normal.

“...SANS?” another voice said, from down the path, causing Papyrus to look up sharply. Somehow, it was his own.

 

Papyrus had just wrapped up another patrol and recalibrated all of his puzzles that day; he was a persevering skeleton, after all. It was now time for him to check up on Sans, and see how his brother was getting along with his work. This was more out of wishful thinking, as Papyrus fully expected to catch Sans napping, or lazing about rather than working, or perhaps gone altogether, a sign that he was probably slacking off someplace else, like Grillby's.

He did not expect to hear himself yelling for Sans when he got to Sans's station, which was wholly deserted with nary a sign of his brother. Last he knew, echoes weren't pre-emptive. They were post-emptive.

The lanky skeleton continued down the road with only moderate concern. Perhaps an echo flower had somehow taken root? In the snow? Where no echo flower had taken root before? With an unusually loud voice for such a flower...? The likelihood of that was not very great, Papyrus thought.

Still, even a misplaced echo flower would have seemed normal compared to what Papyrus found at the end of the road. As you may have supposed, my good reader, what Papyrus encountered by the door to the ruins was none other than himself. Well. A skeleton who looked a great deal like himself. Albeit with sharper teeth. And an angrier mien. And a distinctly more villainous fashion sense- tall, pointy red boots with black buckles, a black top with pointy shoulder-pads, elbow-length red gloves, and a tattered red cape.

The other Papyrus looked just as surprised as Papyrus did, and much more irritated.

“ERM. HELLO...?” Papyrus said. The other skeleton continued to stare- or glare, it was difficult to tell with people who had neutral expressions that angry- at him with blank shock.

“WHO ARE YOU, STRANGE SKELETON?” Papyrus continued irrepressibly. “ARE YOU LOST, PERHAPS? WELL, NO FEAR! I, THE GREAT PAPYRUS, SHALL ASSIST YOU!”

He struck his usual dramatic pose, standing so that the wind would make his scarf billow dramatically. The stranger was definitely glaring now; the malevolence in his expression deepened, as did the angle made by his brow ridges. Papyrus began to feel a bit worried. He had the impression that the other skeleton didn't mean well.

“WHERE IS THIS?” the stranger demanded suddenly, startling Papyrus. His voice sounded just the same as Papyrus's, although it had a fiercely wrathful tone that Papyrus hoped would stay well away from his own.

“THIS IS THE DOOR TO THE RUINS!!!” Papyrus said helpfully. Perhaps the stranger was merely upset over finding himself in such a predicament. “AND DOWN THE ROAD IS SNOWDIN TOWN, STRANGER!”

The other skeleton's orbitals flashed with outraged disbelief (in spite of being entirely dark, like Papyrus's; the curious ways of skeletons are, perhaps, best left unpondered). Yet, familiarity showed in his expression.

“WELL. I DON'T SUPPOSE YOU'VE SEEN ANOTHER SKELETON AROUND HERE?” Papyrus asked. Perhaps he could defuse the situation by changing tact.

But to Papyrus's consternation, this just distressed the stranger even more; he raised an arm and pointed an accusatory finger at Papyrus.

“...SANS. YOU'RE LOOKING FOR SANS, AREN'T YOU?” the black-and-red-clad skeleton said in a disbelieving tone. He was shaking.

Papyrus really wasn't quite sure how to react to this. The other skeleton looked like him, sounded like him, was familiar with Snowdin, knew Sans's name…

“YES...?” Papyrus said uncertainly. Then, he brightened. “ARE YOU MY EVIL TWIN?” Then, his face fell into confusion. “WAIT… DOES SANS ALSO HAVE AN EVIL TWIN???” He boggled. “WOWIE!!!”

 

Evil twin.

That would explain a lot, and now that the idea was out in the open Papyrus really couldn't think of a better alternative. One part of him was pleased by this turn of events. If this weirdly outgoing… version of him could tell enough about Papyrus to know he was evil just by standing in his presence, then he was clearly on the right track. It was kill or be killed in the world he knew; evil really was an excellent descriptor of it. Was this a “good” universe, then? Papyrus scoffed. How pathetic.

“AN EVIL SANS…” the other him said, eyes goggling. “THE SHEER SLOTH MUST BE NIGHTMARISH…”

Papyrus smirked in spite of himself. He was getting far too caught up in this insipid conversation for his own good. “YOU HAVE NO IDEA. BUT I ALWAYS MAKE SURE HE GETS ENOUGH WORK DONE TO STICK AROUND. NO ONE IS A BETTER TASKMASTER THAN THE GREAT PAPYRUS!” he said proudly, raising a fist to his chest and posing so the wind would snatch at his cape. His brother would probably never be able to entirely ditch his slackerly ways, but he was generally pretty obedient, which was good. Undyne would tolerate nothing less, and she would brook no failure. Failing Undyne didn't bear thinking about.

His… “good” twin's expression changed briefly at that remark; he looked uncertain. No… he looked distressed. He covered it up quickly, though. Papyrus glowered at him. What was so distressing about that? It was just the way things were.

“AND THE PUNS… I CAN JUST IMAGINE THE AWFUL, DISGRACEFUL, TORTUOUS PUNS…” the other Papyrus's expression became boggled again, before shifting to thoughtfulness. “ACTUALLY, I CAN'T.”

“...PUNS?” Papyrus echoed, without meaning to. The Sans he knew wasn't one for puns. Neither of them were. Years ago, sure, Sans had used to crack them and other bad jokes all the time, back before everything changed, but he'd stopped. Papyrus just assumed that his tastes had improved, and had been quite glad of that; after all, Sans's puns in particular always got under his non-existent skin, and such foolish behavior was unfit for a sentry of the Royal Guard. Now they mostly just traded insults.

“D-DOES ANYONE ELSE HAVE AN EVIL TWIN?” his counterpart asked. His attempt to conceal his nervousness was pathetically transparent. “LIKE UNDYNE? ARE YOU FRIENDS WITH UNDYNE'S EVIL TWIN, BY ANY CHANCE?”

Undyne? A friend? Papyrus was taken aback for a moment (that was happening a lot in this conversation; it had begun to grate on Papyrus's nerves).

Undyne was the head of the Royal Guard, and she was not Papyrus's friend. She was the person who oversaw Papyrus's progress, yes. She was also the person who, hopefully, would someday give him a place of power in the Royal Guard. She was someone who he looked to for approval. But a friend? No. No, Papyrus didn't have any of those. No one in the Underground did anymore. He hadn't heard anyone be described with “friend” for a long time.

A friend…

Meanwhile, his counterpart was still nervously babbling.

“SAY, IF YOU'RE MY EVIL TWIN, DOES THAT MAKE US BROTHERS, TOO? WHERE DID YOU COME FROM? WERE YOU HIDING IN THE RUINS ALL THIS TIME? AND WHY WERE YOU SHOUTING FOR SANS?”

Papyrus's opinion of his counterpart, not high to begin with, was rapidly diminishing, as was his patience. What was this other him trying to do with all of this pointless conversation? He'd been thrown for a loop more than once already- was his counterpart trying to get him to lower his guard? The dark skeleton's orbitals narrowed even further. Well, it wouldn't work.

“I WASN'T HIDING, YOU SCATTER-BRAINED IMBECILE. THE GREAT PAPYRUS NEVER HIDES!” he said, with angry indignation. This was a point he felt was in desperate need of clarification. Cowardice was Sans's business. “THIS IS NOT MY WORLD.” he continued disdainfully. He resolutely ignored his counterpart's response, which he had decided was more disarming doggerel.

No, this most certainly was not the Underground Papyrus knew. But what was it? Thoughts rose to his mind unbidden. A world where Sans still told puns? A world where Undyne was his- his _friend_? How was it even possible? It sounded like something from before– Papyrus realized he had started shaking again. It wasn't possible. This- it was all some sort of trick. Right.

He gritted his teeth and clenched his fists. He didn't have time to waste on the contemptible weakling who shared his name. He had to find Sans and _stop thinking_ about this stupid, stupid world and its pathetic inhabitants.

His counterpart was still talking at him. Probably more lies. Something about being shocked by a Sans that didn't make puns.

“SHUT UP.” Papyrus commanded angrily, startling his counterpart.

“UM. SORRY??? I WAS JUST-” the other him said, giving Papyrus a blank look of surprise devoid of the fear he'd been expecting.

“SHUT UP!!!” roared Papyrus as he suddenly charged, bones rising from the snow, to silence the lies.

**Notes for the Chapter:**

> The fun part about writing Papyrus is the semi-flowery and often silly language he uses. I always love opportunities to use a phrase like "post-emptive" or words like "doggerel" and "mien".
> 
> For the longest time, I had a lot of trouble getting Underfell Papyrus to do what I wanted him to do- namely, get upset and lash out. He and Undertale Papyrus kept wandering down tangentially-related conversations about Sans and puns and whether or not being evil twins made them related, and generally acting a lot more civil/calm than I wanted. Like, UF!Papyrus! Stop yakking and start attacking!
> 
> Chapter 4 will be here on Saturday.


	4. Chapter 4

**Summary for the Chapter:**

> In the world of Underfell, the two Sanses hunt for answers, and each other.

**Notes for the Chapter:**

> This one starts with Underfell Sans, where we left him back in Chapter 2.

The faint footprints began in the middle of the room, or so it seemed to Sans. There was no trail of footprints leading to the middle of the room, only from it. It looked as if the intruder had poked around his room, then turned and left through the door.

How had the slippered monster gotten into his house, and then into his room? He was sure he had locked the door. It was always kept locked. And how had the blasted thing gotten inside so quickly? Was it following him? The thought of an enemy with who knew how much power, watching his every movement and lurking around his home, filled him with fear. Why was it targeting him? What had it done to his brother?

Sans stared at the door for a moment, papers still clutched in his trembling hands, paralyzed by fear and indecision. A brief memory flashed in his mind, unbidden, of a different time, when a much younger Papyrus had wandered off when out of Sans's sight. He'd spent the whole day searching for him. The small skeleton exhaled slowly. What was happening was entirely unprecedented; he couldn't afford to hide himself away and ride it all out anymore, safe in the knowledge that anything bad would come undone soon enough. Whatever the intruder was, it would pay for messing with them.

He set the papers aside, and then burst out into the hallway.

“W h o's   t h e r e?” he growled menacingly, eye sockets dark, slowly scanning the wide space before him.

He was alone. The intruder was already gone.

Sans snarled in frustration. What sort of sick game was this thing playing at?

He closed his eyes. What _did_ he know about this enemy? Quickly, he came up with a list of facts. It obviously had some power over space, if Papyrus's absence and his unpleasant shortcut was any indication. It knew something about him; at any rate, it knew enough to find Sans's room specifically at the skeletons' house, and that alone spoke volumes. It also wore slippers, which, if nothing else, told him that there was definitely a physical person behind all of this.

A disturbing thought lurched into his head. He did know of one person who knew how to manipulate space, and who knew a great deal about Sans and Papyrus. Well, he hadn't known about the Snowdin house last Sans knew, seeing as he had… _gone_ before they'd moved, but it didn't seem to be that much of a stretch to assume that he had ways of figuring these things out.

Another phantom chill ran down Sans's spine. That was shaping up to be the likeliest possibility, and it frightened him. The only hole in the theory was the pair of slippers, which really were quite perplexing. They didn't seem like his style, and were also too small. He pushed the idea out of his mind, resolved to not jump to unsettling conclusions until he had more evidence.

He had to hunt for the intruder. If he was an enemy, and he was trying to spy on or terrorize himself, where would he go, now that he'd left the house? Sans immediately went back down to the workshop, and found that nothing in the mess he'd made of it had been disturbed. He then went back to his room and retrieved the papers, heedlessly shoving them in his pocket as he wracked his brain for possibilities. Grillby's? One of his stations? He would have to check them all, as he had quite a few; being able to work in several places, even if it wasn't much work in any of them, was one of the few reasons why Undyne let him stick around.

Sans concluded that if he was the intruder, he would probably go to Grillby's next. Standing in his room, he took a step forward, and his foot landed in the snow in the forest beside the back wall of the seedy place. The reverberations in space had finally stopped. Now it was time to watch, and wait.

 

The other Snowdin Town, in comparison to Sans's home, felt not so much unwelcoming as downright hostile, and no one had even seen him yet. If they had, Sans was sure they would have given him a very cold reception. There tension in that town so thick it was almost tangible, clinging to the very air itself like humidity in Waterfall after a heavy downpour. The buildings looked as if they had once been the same as those Sans was familiar with, before being taken in a different direction.

Many of them had been painted in dark colors, in coats so crisp as to give them a sharp, unfeeling look. A few were in bad shape, showing signs of slowly falling into disrepair, with missing shingles and hanging eaves. Some of these had lights on inside, and generally seemed despondent, as if their owners had given up on taking care of them. Others were uninhabited, for what reasons Sans did not dare to guess, with dark, hollow-seeming windows, and cracking black paint peeling away to show the familiar wood underneath. At the entrance to the town was a wooden sign, with “Snowdin Town” painted on it in plain black letters.

From what Sans could see, hidden as he was by the entrance to the tunnel, in the shadow cast by the inn, the inhabitants were even more unsettling, because Sans recognized them. These people were his neighbors, for crying out loud, and seeing them like this- most of them infinitely more irritable, others acting mean and petty, a few downright thuggish- was a new kind of disturbing.

The one difference that was more perplexing than disconcerting was the aesthetic (for lack of a better term). Everyone wore black with red and yellow, as if they were _trying_ to look edgy and villainous- which was probably the case, he realized with some bewilderment, if everyone's demeanor was anything to go by. In general, the look of everything- the buildings, the people, all of it- made him think of a barbecue gone wrong, and look for all the world like ketchup and mustard spilled over so much char.

He stepped back a little farther into the shadows, wondering what to do next. He had to figure out what had happened and how to undo it, and to do that, he would need to do some research- and to do that properly, he would have to go back to not-his house. But the monster that looked like him was still there, and Sans didn't want to risk being noticed by going back.

Eventually, he settled on satisfying his curiosity by checking out the other version of Grillby's. This quickly proved to be a mistake, which you, my good audience, have probably been expecting, as when he took a shortcut to the trees behind the place, he discovered he was not alone.

His counterpart was already there, and quickly started to turn around to look at him when Sans took a shortcut away again, this time back to “his” station in the woods. Bizarrely, it looked tidy and well-tended, as if the person who worked there actually _worked_. He couldn't help but notice, with slight dismay, that there were no packets of ketchup, or any other condiment, in sight. Sans kicked at some snow, idly wondering if maybe his counterpart had hidden them.

But even that brief action was too long a delay, for not a second after, he heard the distinct if quiet sound of someone taking a shortcut.

“W h a t-”

Sans left.

He was in the workshop under the house, which looked like it had been hit by a tornado; he had a feeling the notes he needed were already gone. Sans chanced a glance where the machine might be- it was there, or else something of the same size and shape was, hidden beneath a dusty tarp that clearly had not been touched in ages. Where were the notes? He needed to get his hands on them. As good as he was at quantum mechanics and theoretical physics, he couldn't sort out this entire mess in his head.

Scritch.

“-h a v e-”

Now, Sans chose a location at random; the bank across from the fishing spot by the river, just outside the town. Hopefully, by going somewhere unexpected, he'd buy himself some time to think and plan where to go next. Suddenly, his phone rang. He fished it out of his pocket and held it out at arm's length for what squishier monsters would call a heartbeat, looking at it with some alarm. Then, hesitantly, he held the phone up to his head to take the call.

This was a decision he instantly regretted, as first and foremost the- _thing_ on the other end, if indeed there was anything there, only made horrible, distorted noises that vaguely resembled screaming and roaring, and secondly, it evidently had bought his counterpart enough time to catch up to him, as he materialized on the far bank.

“-y o u-”

“nope.” he muttered under his breath.

Sans had no intention of being caught by an angry thug who happened to share his face, and the other him clearly had no intention of letting him investigate in peace. Surely he couldn't keep guessing Sans's movements forever.

But then again, Sans mused as he momentarily stopped off at the thankfully empty snowball course before stepping out onto the bridge near town, was he really getting anything done, teleporting hither and thither across Snowdin like this? If he was in his counterpart's slippers- did the other him wear slippers? He hadn't gotten a good enough look at him to see- he would have hidden the notes he needed already. Heck, he would have scattered them across the Underground, and stuffed a few in his jacket with the ketchup for good measure.

Sans left again. Perhaps he needed a new approach.

He'd chosen the door to the Ruins as his next destination, thinking that perhaps it might be a good idea to backtrack, and throw his counterpart off. But instead of heading off again, he stood and waited.

_3… 2… 1…_

“-d o n e ?”

“hello to you, too.” Sans said.

 

**Notes for the Chapter:**

> Underfell is populated with angry middle schoolers.  
> On a different note, what's a good plural for Papyrus? Papyruses? Papyri?
> 
> Chapter 5 will be headed this way on Tuesday.


	5. Chapter 5

**Summary for the Chapter:**

> The situation with the Papyri heats up.

**Notes for the Chapter:**

> Undertale Papyrus is a cinnamon roll. Underfell Papyrus is, or was a cinnamon roll, but in the words of my friend Good-Golly-Gee, he got burnt in the oven.

Most of the first flurry of bones went wide, as his evil twin's haste overcame his aim, and it wasn't terribly difficult for Papyrus to dodge them. His villainous double snarled in frustration, and stomped his booted foot hard. At once, Papyrus felt heavier, and glancing down, saw that he was tinged blue. This did not concern him much; the Great Papyrus was an excellent jumper. A little extra gravity wouldn't hold him down!

“DO WE HAVE TO FIGHT?” Papyrus asked. “I SUPPOSE GOOD TWINS ALWAYS FIGHT THEIR EVIL TWINS IN STORIES, BUT-”

Suddenly, Papyrus froze up stock-still, to let a pale blue bone pass through him, then jumped in place to avoid a shorter plain one. Several more such pairs raced at him, but he dodged them all. Papyrus knew his own magic pretty well, although he usually took things more slowly.

He also had the feeling that this fight was about more than just good twin versus evil twin. His evil twin was obviously angry, but more than that, he had also seemed very agitated before he attacked. Papyrus couldn't quite put his finger on why, but perhaps his evil twin was not beyond persuading, and could be convinced to stop the fight.

A speedy wave of bones that changed in size, some growing taller as others became shorter, advanced at him. The tall skeleton leapt over these, too. The other him glared at him most menacingly, but this did not worry Papyrus; it was his counterpart's default expression, after all.

“STOP THAT!!! PLEASE! LET'S NOT FIGHT.” he said plaintively.

Lamentably, his counterpart did not heed Papyrus's appeal, and with a roar sent out another row of size-changing bullets. Papyrus jogged down the line to where the bullets were shrinking, was able to step over the shorter ones. He rattled his bones.

“CUT THAT OUT!!! I DON'T WANT TO FIGHT YOU! AFTER ALL, I MUST GO FIND SANS!!!” he said, feeling rather bonetrousled. “SAY, WEREN'T YOU ALSO LOOKING FOR SANS?”

His counterpart's glower deepened at the mention of Sans. He clenched his fists angrily. Then he bellowed, “RRRAGH! FIGHT BACK, Y-YOU PATHETIC WEAKLING! FIGHT BACK!”

Reluctantly, Papyrus moved a few blue bones his counterpart's way. Hopefully he wouldn't be able to keep attacking if he had to stay still.

When the bullets dissipated, his counterpart stared at him with disbelief. Papyrus tried to keep eye socket contact; was his counterpart ready to end the fight? For a moment, neither skeleton did anything.

“UN. FATHOMABLE.” his counterpart muttered. Then, he said aloud, “YOU ARE THE MOST FEEBLE-MINDED… HOW DO YOU LIVE??? HOW HAVE YOU NOT BEEN KILLED??”

The villainous version of Papyrus went straight back to attacking, this time shooting a fast-moving line of bones towards Papyrus from either side of him. It was a bit trickier to dodge, as he had very little time in which to jump, but the tall skeleton managed. He was the Great Papyrus, after all!

An idea came to him, a truly brilliant idea. Nothing that his counterpart had done was unfamiliar to Papyrus. Certainly, his evil twin was going much faster and fighting harder (and wilder) than Papyrus liked to, but everything he had done was well within Papyrus's capabilities. Were he to fight just as hard, Papyrus surmised, they would be on perfectly equal footing. Perhaps that was the way to stop the fight- if his evil twin could be shown that they were matched in strength, he might see the foolhardiness of continuing the duel, as surely neither of them would be able to best the other. Well. Papyrus probably would triumph in the end, if only because his counterpart, having fought hard from the beginning, would tire out first.

It was worth a shot.

Papyrus turned his evil twin blue, and cackled. “NYEH HEH HEH!”

He let his magic rush out at the pinnacle of its power, and many pairs of bones went out to meet his opponent, one above the other, forcing his evil twin to twist and jump to get through the gaps.

 

Papyrus's fury burned hot at his counterpart's audacity. Had the insufferable fool actually been holding back? What could possibly make someone do something so horrendously stupid as to not use their full strength against an enemy, especially one that actually meant them harm? Good twin, nyeh!- pathetic twin was more like it.

The two skeletons' bullets were flying thick and fast, scarcely letting up save when the opponents had to pause from attacking in order to dodge. For a moment, Papyrus was glad that Sans was not there, for he surely would have been obliterated in an instant if he got caught in the crossfire. Briefly, he considered getting in close; monsters, at least in his world, were adept at dodging projectiles from far away, but no one could avoid a sharp strike at point-blank. A quick jab at the right distance might end the fight quickly… but no, if he charged in his opponent would have time to prepare a short-range attack before he did. Better to stay away and hope one of his bullets connected.

He cleared a group of oncoming bones in a bound, and found that he had to pause to catch a quick breath (while as a skeleton he had no lungs to fill, breathing was a good way to re-focus one's magic after a lot of exertion). Anger bubbled up most unpleasantly within him, together with a hint of fear that he was unable to quash. Was that why his enemy had held back? To tire Papyrus out while his counterpart conserved his strength?

It was time to put an end to the other skeleton's impudence. Papyrus readied his special attack.

A single, small, floating bone bullet appeared in front of him. His counterpart's eyes widened, and Papyrus smirked at his apparent distress. The bullet extended, growing longer and thicker, and once it had extended farther than Papyrus was tall, he reached out and took it. Then, wielding it like a baton, he spun it in his hand and hurled it, sending it whirling at the other skeleton.

His counterpart hastily ducked, hands held up protectively over his head. The baton grazed him and came back to Papyrus at the end of its circular course. He deftly grabbed it out of the air and cackled himself, triumphant over (finally) scoring the first hit.

“NYEH HEH HEH HEH!!!”

Not to be outdone (or so Papyrus read it), his pathetic twin prepared his own attack, although now he looked rather worried. Soon, two batons were rounding the battlefield, the Papyruses jumping and weaving and even taking the time to show off a bit. Papyrus got clipped on his right arm when his counterpart flung his baton around and it circled twice, and he got in another hit when the other skeleton failed to jump high enough in time to avoid the twirling bone.

He took a deep breath as his attack came back around to him, hoping the other skeleton wouldn't notice. His pathetic twin was starting to look a bit worn himself, and held his baton upright in the snow like a staff.

“NYEH HEH HEH!” his counterpart cackled, filling the lull in the conversation with his chatter. “AS YOU CAN SEE, WE ARE EVENLY MATCHED!”

Papyrus had to give him that. Once he'd finally gotten his opponent to go on the offensive, everything that had been thrown at him had seemed just as strong as his own magic.

“TH-THIS FIGHT IS POINTLESS!” his counterpart continued. “AND SO I, THE GREAT PAPYRUS, SHALL SHOW YOU MERCY!!! I AM THE GOOD TWIN, AFTER ALL.”

Papyrus stared blankly at him in shock. Mercy? He was fighting to- what was he fighting to do? He meant the other skeleton harm, at any rate, and yet his pathetic twin was giving up his own offensive? What kind of person did such a thing? It was unfathomable.

His counterpart was probably lying anyway, he told himself. It would not do to fall for such a ploy.

Regrettably, he had to go back to using regular attacks; keeping up his special attack was taking up too much magic and not doing as much damage as he would like. It was going to wear him out. So, he let his baton dissipate, and sent another wave of bones at his opponent.

“REALLY, I THINK IT WOULD BE BETTER IF YOU ACCEPTED MY MERCY...” his counterpart said, breathing audibly louder as he flung himself over the bullets.

Who was this?, Papyrus wondered. In spite of Papyrus attacking him, in spite of clearly trying to harm him, in spite of screaming at him and turning down his mercy, and in spite of the other skeleton's own clear fear, he was still determined to spare Papyrus. Why? What kind of bizarre, twisted world was this that had made his counterpart turn out like this- friendly, merciful…

Why hadn't _he_ turned out like this?

Anger churned up within him, although he could not place what, exactly, he was angry at. This was not the time for these questions. That time would never come.

He shut his eyes against them readied his next attack.

 

Papyrus didn't bother to counterattack. He didn't want to fight anyway, and would rather focus his energy in getting out of the way. His attempt to defuse the situation had only drawn it out further. He didn't understand bad guys. Why wouldn't his evil twin just let Papyrus spare him? Why did he keep fighting? What did he want?

A lightbulb went off in his skull. They were counterparts well-matched in appearance, magic, voice, and even laugh. Perhaps they were matched in motivation, too. What did _he_ want?

He wanted to be in the Royal Guard. No, he had to go beyond that. He wanted Undyne to be proud of him. But… his evil twin hadn't really said much about Undyne, so better go with something else. He wanted friends. Did bad guys have friends??? That could be his back-up idea; he wasn't terribly certain of it. He wanted respect, recognition, for people to look up to him. There was something he could work with.

After clearing yet another bout of bones, Papyrus stood up straight, one hand to his chest, letting the wind catch at his scarf.

“IT IS CLEAR THAT YOU ARE A SKILLED AND POWERFUL VILLAIN!” he announced. His counterpart paused; he had his attention, at least. “TRULY, YOU ARE WORTHY OF BEING THE EVIL TWIN OF THE GREAT PAPYRUS!”

For a split-second his evil twin looked surprised, but he quickly covered it up.

“I SHOULD CERTAINLY HOPE I AM AT LEAST THE MATCH OF YOU. I SHALL BE THE GREATEST ROYAL GUARD THE UNDERGROUND HAS EVER SEEN!” he said contemptuously, although Papyrus could see him smirk in spite of himself. Papyrus inwardly sighed with relief.

“I, TOO, DESIRE TO BE A GUARDSMAN,” he proclaimed. “FOR WITH THAT POSITION I WILL OBTAIN ALL THE THINGS I SO RICHLY DESERVE! RECOGNITION… RESPECT… ADORATION…”

His evil twin nodded in agreement. “EVERYONE SHALL FEAR ME WHEN I COMMAND THE ROYAL GUARD. THEY WILL TREMBLE WHEN THEY HEAR MY VERY NAME!!!”

Papyrus decided to ignore that last part. “BUT!!!” he said. “YOU CANNOT DO ALL THAT IF YOU ARE HERE IN THIS WORLD.”

His counterpart was startled out of his ego trip, and fixed Papyrus with a suspicious glare. Did he know how to do anything but glare?

“I PROPOSE A TRUCE.” Papyrus announced. “WE WILL SET OUR DIFFERENCES ASIDE WHILE WE WORK TO GET YOU BACK TO YOUR OWN WORLD! THEN I, THE GREAT PAPYRUS, SHALL BECOME THE BEST GOOD ROYAL GUARD I CAN BE, AND YOU MAY BECOME THE WORST BAD GUY GUARD YOU CAN BE!!!”

His counterpart continued to look suspicious, and Papyrus waited with bated breath. Then, suddenly, he noticed something coming out of the forest. It was short, and not so much squat as it was grotesquely flattened. His evil twin noticed it too, and gave it a look that fell somewhere between horrified and disdainful.

“Ka-sigh...” the newcomer said, in a voice that seemed to be perfectly tuned to be obnoxious as possible, glancing about with beady, shifting eyes.

The two skeletons exchanged a look. Then, the evil Papyrus's eyes widened, and he leaned back a ways, holding up a hand, looking off somewhere behind Jerry, who turned to see what was being reacted to. Instantly, Papyrus and Papyrus took off down the road to Snowdin Town as fast as their legs would carry them. 

**Notes for the Chapter:**

> This was another tricky chapter to write, partly because Underfell Papyrus never behaves himself, and also because I knew that if both Papyri were fighting their hardest I HAD to include their special attack. Hopefully this one makes sense- I got the idea from my friend Kittfox, who thought that a baton fit Papyrus's showy nature.  
> Underfell Papyrus's fighting style is described as being wilder than Undertale Papyrus's because he's self-taught; Underfell Undyne doesn't train him and doesn't intend to let him into the Royal Guard, because she's afraid he'll use his considerable power to supplant her.  
> Also, I hope that all the dodging they do doesn't break anyone's suspension of disbelief- in-game, I know that none of the monsters bar Sans dodge, but if they're fighting other monsters are they seriously just gonna stoically take a bunch of bullets to the face?  
> I did like getting to showcase Papyrus's more perceptive side here- my headcanon has always been that he's much more insightful/aware of things than he lets on. Underfell Papyrus, not so much.  
> Underfell Papyrus fights more wildly than Undertale Papyrus because Underfell Undyne hasn't been training him; she's aware that he's a powerful monster, and doesn't want to let him become so powerful that he might supplant her.
> 
> Chapter 6 will probably be up some time Saturday.


	6. Chapter 6

**Summary for the Chapter:**

> Sans and Sans get along about as well as oil and water.

**Notes for the Chapter:**

> Starting from the perspective of Undertale Sans, at the door to the Ruins where we last saw him in Chapter 4.

Sans stood calmly, hands in pockets, and faced down his counterpart. The other skeleton glared at him fiercely, but as the moment dragged on his expression changed to what was best described as disturbed surprise. He wore a black winter coat, rather than a blue hoodie, a red turtleneck instead of a white one, and black pants striped with yellow. Instead of comfy slippers, the other Sans had black boots. Sans pitied him for that, but more unsettling were his counterpart's teeth. All of them, even the gleaming gold one, had been sharpened, giving his counterpart's rictus grin an almost shark-like appearance. He hoped that it was a natural thing and tried not to think too hard about it.

“who are you?” his counterpart said suddenly, expression screwed up into a malevolent scowl.

“the short answer? you.” Sans said with a shrug. “the long answer? also you, but a different version, you might say. like… an alternate universe version of you. or are you just an alternate universe version of me? heck if i know.”

His counterpart's eyes narrowed.

“an alternate universe. really.” he said. Then, his eyes darkened, and he took a step forward. “how did you get here?!” he demanded.

“uh…” Sans thought for a moment, then shrugged again.

“T e l l   m e !” his counterpart shouted angrily, baring his sharp teeth. His pupils had reappeared, and his left eye was wreathed in a harsh red glow. He took another step forward, and a single monstrous, red-eyed skull manifested behind him.

Sans felt a bit heavier, but not too worried. He raised a brow ridge and cocked his head. Just the one Gaster Blaster? Curious. His counterpart was clearly doing everything in his power to intimidate him, but Sans got the impression that there wasn't much that he could actually _do_. Considering how many Gaster Blasters _he_ could summon at once, bringing out just one seemed a bit… underwhelming, somehow. That, and his counterpart was sweating nervously. He was putting up a facade, then, trying to cow Sans.

Well, Sans was having a very strange and trying day, so he was having none of that. Keeping his expression as chill as possible, and not even taking his hands out of his pockets, he tapped into his own magic. His vision became tinged with a flickering blue, and he allowed three Gaster Blasters of his own to appear around him.

“whaddya know. we do have more in common than just our good looks.” he said lazily.

His counterpart was stunned, and stared at Sans with wide-eyes for a moment before his face contorted into an expression of helpless fury. Reluctantly, he withdrew his single Blaster, and the red glare in his eye socket burned itself out.

“what do you _want_?!” the other skeleton asked. “w-what have you done?!”

“i tried to take a shortcut. felt pretty off. so i ducked back into normal space but wound up here, instead of the ruins i know and nap at.” Sans said, dismissing his own Blasters. “didn't you notice it, too?”

His angry edgy doppelganger nodded. “yeah. b-but if you didn't cause it, then what did? and where's papyrus?!”

So there _was_ a Papyrus in this world. Sans involuntarily imagined a thuggish, perpetually angry, sharp-toothed Papyrus with a thirteen-year-old's fashion sense and had to suppress a shudder.

“honestly, i have no idea. though i have a feeling you're sitting on all the data.” Sans held up his hands. “willing to share? I think we both want me back in my world, and your brother back here, so… whaddya say?”

The other him was still glaring at him distrustfully.

Sans saw something moving in the distance, past the other skeleton. It was splotched with red and black, like it had upended cans of paint on itself in an attempt to match the general aesthetic of the alternate world. In terms of shape, it was lumpy and squashed, bringing to mind an old rotten gourd, or perhaps a particularly hideous UFO, with a scrunched-up mouth shaped like an inverted W poorly scratched into something with a knife, and more-or-less triangular eyes that gave the impression of seeking out people to harass as they shifted back and forth.

He took a hand out of his pocket and pointed at the newcomer. His counterpart turned to look at it, and an expression of intense exasperation took over his face. _Jerry_.

The Sanses exchanged a look, and wordlessly came to an understanding: They could get on each others' case later. This was a more pressing issue. Before the oncoming thing could get a step closer, both skeletons promptly teleported away.

 

In the next minute, the black-jacketed Sans was standing by the black table with papers on it in his own house, while his blue-clad counterpart was facing him, back to the door. His counterpart seemed to be looking at the rock they used as a paperweight. Sans raised a brow ridge.

“uh. it's a rock. what's so interesting about it?” he asked.

“eh. just, that's a pretty gneiss rock. it's a shame that you're taking it for granite like that.” blue him replied.

Sans's jaw clenched. What was _wrong_ with his counterpart?

The other skeleton must have seen his unimpressed look, because he pressed on, “of quartz, i shale have to overlook that. metamorphically speaking, schist happens; can't let myself get worked up over every problem i happen to dig up…”

He knew it wouldn't intimidate his counterpart into silence, but Sans glared at him nonetheless. They were terrible puns. Absolutely awful. He did not find them funny in the slightest. He was not resisting the urge to laugh at them. Nor was he wishing he could still come up with off-the-cuff puns like that.

Infuriatingly, his counterpart just smiled even more widely when he saw Sans's irritation.

“do you want to get out of here, or not?!” he asked angrily.

The blue-clad skeleton heaved a sigh and rolled his eyes. “yep. so. got any hypotheses about what happened? why space is out to lunch?”

“…no. figured it was all your fault.” Sans replied peevishly. “didn't know about this alternate world crap until you decided to stop running.”

“have you looked at any data or reports on space-time?”

“no. you broke in before i had a chance to.” Sans said.

His counterpart closed his eyes for a moment. “all right.” he said at last. “do you at least have the papers with you?”

Sans popped up into his room, with the intention of grabbing the papers from off his drawer and going right back to where he'd been, but his counterpart's reflexes were just as good as his own, and Sans hardly had time to start reaching for the papers before the other skeleton showed up beside him.

His counterpart gave him a questioning look but said nothing. Sans wanted to blast him to smithereens, but knew that he'd never get away with it without being dusted himself. Instead, he shoved the papers at his counterpart and ground his teeth.

“huh. i recognize most of these.” his counterpart remarked amicably.

“great. get on with it.” Sans said, tetchy. This other him was a real lout, trying to act all superior and show Sans up, and it was getting on his nerves. He took his bottle of mustard out of his jacket and took a quick swig from it.

“welp. none of these say anything about the sub-levels of the fabric of space destabilizing in a singular event.” other-Sans said. He held up a report from the sheaf of papers. “this one discusses the possibility that excessive teleportation might cause sporadic reverberation, but the calculations all predict it would be continuous, and that's never happened before. so i can't say what caused this.”

Sans groaned in aggravation.

“yeesh, that's not the end of it.” his counterpart said. Sans's teeth-grinding became audible. “i've got an idea. i'm just gonna need more information. after i first got here, i saw you poking around where i'd been standing. thought i heard you mention paps.”

“what of it?” Sans growled defensively.

“were my footprints over his?”

“uh- yeah, they were. so?”

His counterpart nodded, ignoring Sans's belligerence. “hoped so. well, i still dunno how it happened, but i think the initial destabilization occurred exactly when i tried to take a shortcut back in my world. then, when i ducked out of sub-space, instead of going back to my world, the reverberations- i dunno- pushed me over here somehow. maybe a boundary between the worlds dissolved, so i just crossed over.”

“what has this got to do with papyrus?” Sans growled.

“somehow, your brother was standing in the exact spot in this world that i'd been in my world. so when i came over here, i displaced him.” his counterpart said.

Sans's orbitals went dark and widened in an expression of angry surprise. “Y o u   d i d   w h a t ?!”

“better than overlapping, i suppose.”

Sans had had enough of the other skeleton's nonchalance. “W h e r e   i s   h e ?!   T e l l   m e,   y o u   f r e a k !”

In spite of his counterpart's earlier show of casual power, Sans let his vision turn crimson once again.

“look, i'm sorry about your brother, but i don't know. we probably just switched places.” his counterpart replied. He looked thoughtful for a moment. “hey. is your brother like you? y'know, pointy teeth and bad attitude and everything?”

Sans simply glared at him, inwardly debating if it would be worth the risk to just summon a Gaster Blaster and get rid of the problem. He really, _really_ wanted to blast the other skeleton to smithereens.

“i'm taking that as a yes. right. we need to swap 'im back here.” His counterpart winked his right eye, and his left one began to fill up with blue. Reluctantly, Sans backed down. “have you been working on the machine, too, by any chance?”

 

**Notes for the Chapter:**

> My only excuse is that I'm a geology major. Hope you liked those marblous puns.  
> Am I the only one a tad bothered by the fact that the skelebros in Underfell both have sharp teeth? I suppose if you ascribe to the corrupted file idea it's not that bad, but for the sake of this story Underfell and Undertale were extremely similar (but probably not entirely identical) until something caused them to go off in different directions. So at one point, the Underfell skelebros must have had normal teeth. Do monsters' appearances change as they grow to match how they act because they're made of magic/soul, or...?
> 
> Underfell is populated by pouty children.
> 
> Chapter 7 will be uploaded on Tuesday.


	7. Chapter 7

**Summary for the Chapter:**

> Both sets of brothers try to figure out how to sort out this mess.

**Notes for the Chapter:**

> Sans and Sans bickering always makes for good stuff.

It was clear to Sans from the general state of the dimly-lit workshop that his counterpart from this world had not, in fact, done any work on the machine. The floor and most of the tools and gadgets were snugly covered in cozy blankets of dust and grime. Papers and folders and files, which had been tossed about the room with wild abandon, were still lying where they had been scattered. In the corner, the machine stood covered in its tarp, which had lain there untouched for years. Curiously, a comfy-looking old sofa was pushed up against the wall. It looked a lot like the one Sans had in his own home, except it was gray instead of green; well, it looked gray. Considering how filthy it was, it could easily have once been a different color entirely.

“so, you haven't touched it at all?” Sans asked, eyes closed. He had his work cut out for him, especially since his counterpart probably wouldn't be able to help at all. Then again, considering how abrasive the other skeleton was being, that was probably for the best.

“nope.” the other skeleton said. His hands were balled up in his pockets, and he was fidgeting where he stood, pupils darting about the room, sockets narrowed in irritation.

“back in my world, i've spent years working on this thing; i'll be working from scratch here.” Sans warned, as he carefully pulled the aged and faded tarp off the machine. “heh. as if years mean anything anymore. fixes i make stick around after resets, and who knows how many of those there've been?”

His counterpart shuffled and kicked at some papers behind him. “how much do _you_ remember of those?” he asked.

“eh. they seem to come in cycles, and i remember everything during a cycle. but after we go to the surface, or… the other extreme, things get pretty hazy. if something's happened a bunch, i'll know that it's happened, though i don't always have real memories of it until it happens again, y'know? uncommon events i usually don't remember, but they sorta feel like deja vu when they happen again, and then some fuzzy things come back. and this… i dunno what i'll remember of this after a cycle or two.” Sans replied with a shrug.

“huh.” the other skeleton said, but if his memory worked differently, he didn't say. Sans wondered why he didn't just sit on the sofa; he seemed to be avoiding it. “whaddya want with that thing, anyway? how's it supposed to help?”

Sans balled up the tarp and tossed it to the floor. Beneath it, the machine was in pretty awful shape. A few tubes and components had blown out entirely, and some other pieces were bent horribly out of shape. Thankfully, the screens were still intact.

“it's meant to travel through time-” Sans started.

“i know _that_ , deadhead. how's that supposed to help?” his counterpart snapped, still standing.

Sans took a deep breath. “space and time are connected. making things move through space is simpler than moving 'em through time, though. so maybe i can get it to do some of that.”

The other Sans growled with frustration. “maybe?!”

Sans tapped on the screens. “and these can be used to monitor the local condition of the fabric of space-time. make sure it's safe to screw around with the continuum. even if i can't get the machine to move anything, getting it to show us a stability readout won't be that hard. if whatever-it-was happens again, we'll know.”

“hmm.”

Thankfully, the other Sans lapsed into silence for a while, allowing Sans to get to work. He started by inspecting the circuitry and wiring of the machine, which, fortunately, seemed to be in better condition in this world than they had been in back in his own. If he cut off everything but the screens and their associated sensors from the circuit, he would probably only have to make sure they were safely connected and maybe patch up a few parts before plugging it in. Then, it would either burst into flames, or start scanning space. That would be a good start.

Then, as Sans began isolating the sensory systems, the other skeleton piped up again.

“hey… whaddya think would happen if a reset happened, and you were still here?” he asked.

Sans stopped working for a moment. “i'd rather not think about that.” he said.

“might not be that bad.” his counterpart said genially. “maybe you'd be sent back to your world, like none of this ever happened.”

Sans went back to work. He hadn't known his counterpart very long but he already suspected exactly where this was headed. Other Sans hadn't touched the machine in ages, but he knew of it, and must have known who made it.

“of course, there's always the chance that you'd be erased entirely…” Yep. Just as Sans had figured.

“yeah. that would be pretty miserable.” Sans said lightly. “i'd be gone… your brother'd be gone, no way of getting back…”

His counterpart swore suddenly. Sans glanced over his shoulder, and saw his counterpart- still standing- staring at the wall, looking stunned. When he saw Sans looking, he reshuffled his expression back into its usual anger.

Another tense silence followed, but Sans worked on steadily. The antagonistic conversation had strengthened his resolve; besides not wanting to be erased from space, as he knew how that would probably go, he did not want to put Papyrus through the pain of losing him. And, as offensive as his counterpart was, Sans wouldn't wish for him to lose  _his_ brother in a million years.

“uh, on a less nihilistic note, why'd you call me earlier just to scream at me?” Sans asked. Actually, he had a feeling it hadn't been other Sans who had called him, but he wanted to test an idea.

“i didn't! you _numbskull_.” his counterpart said derisively. “i- what?!”

Sans was snickering. “heh. so you do have a funny bone.” he said, grinning. His counterpart looked at him with a curious mixture of indigence and bewilderment. “wait- did you not…? heh, that's even more _humerus_!”

The other skeleton's eyes narrowed in consternation, and he slapped a hand to his face, groaning in aggravation.

“I   h a t e   y o u.” he said in a low voice. Sans shrugged and kept snickering.

“and i didn't call you. i thought you tried to call me. didn't pick up. figured a lowlife like you wasn't worth it.” the other skeleton continued testily.

Lowlife was a bit rich, coming from a thug like other Sans, or so Sans thought. “well, if you didn't call me, and I didn't call you, then… hmm. think he's behind it?”

Sans heard his counterpart sit down heavily on the sofa behind him. He turned around. The other skeleton looked tense, eyes fixed ahead of him, hands clenched in his lap. A few beads of sweat had formed on his head. More notably, his expression was one of extreme discomfort, and anxiety. His right eye- the one that had glowed red- was closed, and when he saw Sans looking at him he quickly shut the other one and then opened both, before molding his face back into something a little more glare-y.

“don't.” his counterpart said, voice taut. “don't talk about… well. just don't. who cares how it happened?! just get the fricking thing working already!”

 

When the two lanky skeletons ran down the final bridge, and the sign welcoming all to Snowdin Town came into view, the good twin-Papyrus skidded to a halt. The evil twin-Papyrus barely managed to stop before he ran into him, and scowled- well, scowled more.

“WHAT NOW?” he demanded, irritated as always by his counterpart's antics. “WHY HAVE YOU STOPPED?”

“WELL… IT'S JUST THAT… IF YOU'RE SEEN, WELL, I HAVE A FEELING THAT WILL LEAD TO SOME AWKWARD QUESTIONS.” his counterpart said sheepishly.

He had a point. Evil twins didn't just spring out of the grass every day, and drawing attention to himself could only end in supreme aggravation. Papyrus didn't think he could bear dealing with a whole town of pathetic ninnies and weaklings equally insufferable as his counterpart.

“HMPH.” Papyrus said. Then he frowned (more). “WHERE ARE YOU LEADING ME, ANYWAY?” he asked, although admitting that he was following his counterpart's lead left a bad taste in his mouth.

“TO MY HOUSE!!” the other Papyrus said.

“…WHY?”

“WHY NOT? THAT, AND I HAVE MY SPAGHETTI THERE. THIS SEEMS LIKE A PROBLEM THAT NEEDS TO BE TACKLED WITH SPAGHETTI.”

He had a point. Come to think of it, Papyrus couldn't remember the last time he'd had spaghetti; the possibility of pasta made the idea seem much more tantalizing. But he couldn't just _agree_ with his counterpart like that. It wouldn't do for a Royal Guardsman to simply accept an inferior's idea like that- and his counterpart's, for all his foolishness, was most definitely an inferior. Papyrus had a reputation to uphold and appearances to keep up! True, no one whose opinion he cared about was nearby, but that was beside the point. The point was that Papyrus refused to let his Royal Guard attitude slip, so he kept it up all the time. He regularly had to come up with reasons to agree to Sans's suggestions for that very reason.

However, he already had a perfectly good reason to go along with the plan.

“HMM. IF SANS _IS_ HERE, THEN THAT WILL PROBABLY BE THE FIRST PLACE HE WILL GO.” Papyrus said.

“WHICH SANS? MY BROTHER SANS, OR YOUR BROTHER SANS?”

“MY BROTHER.” Papyrus gritted his teeth.

“DO YOU THINK HE MIGHT BE THERE NOW?” The other skeleton looked rather worried, for whatever reason. Perhaps he was afraid that Sans might steal his pasta, or perhaps he was considering the prospect of having to deal with both Sans _and_ Papyrus. Inwardly, Papyrus hoped it was the latter, and it would be well-warranted if it was. Slackerly and cowardly as Sans was, he was good at intimidating people- almost as good as Papyrus.

“PERHAPS.” Papyrus said, reveling in his counterpart's anxiety.

The other Papyrus was very keen to head off after that, awkward questions or no awkward questions, and while it took some time the two skeletons managed to get to the “good” twin's house without drawing any attention to themselves. They did not sneak. The Great Papyrus did not sneak, not like his skulking brother. They were just stealthy.

“NOW WHAT?!” Papyrus asked irritably, once they were inside the house. It was brighter than his own, in part due to the strange colored lights strung up outside. What _were_ those, anyway?

“NOW!!! WE!!!” His counterpart narrowed his sockets in concentration. “HMM. DO YOU WANT SPAGHETTI BEFORE WE START CRAFTING A PLAN TO GET YOU BACK TO YOUR WORLD?”

As impatient as he was to leave this strange and upside-down world behind, Papyrus didn't object. He was disappointed, albeit not surprised, to find that Sans was not, in fact, in the house. Also, he had no understanding how how he had crossed into a different world, and suspected that his counterpart didn't, either; no knowledge plus no knowledge was still no knowledge, and had he not been the Great Papyrus his confidence in their collective ability to get him out of this world would have been nil. As it stood, it was only mostly nil. At least there would be spaghetti.

“VERY WELL.” he said dismissively. His counterpart bustled off to the kitchen, leaving him alone in the main room.

The differences between it and his own house were as apparent to Papyrus as they were to the other Sans (and, my good reader, to you). He was, however, particularly struck by the presence of the green sofa. It looked just as comfortable as he remembered it, from when he had owned it back in his own world.

He stared at it, but did not sit down. The sofa had been removed some time ago, when Papyrus was first trying to get Undyne's attention and apply to join the Royal Guard. Something about it had seemed frightfully ill-fitting for the home of a Royal Guardsman, and he'd had Sans dispose of it. Seeing it- well, a copy of it- reminded him of… of…

Papyrus strode away from the sofa, and glanced out of the window. A few townsfolk were out and about- weak, soft, going about their pathetic and petty business. It was so different from the town he was familiar with- tough, strong, powerful. Harsh. Cruel…

Why was it like this?

He felt cold, and he realized he was shaking again, but he kept looking out of the window.

**Notes for the Chapter:**

> Originally, Chapter 7 was going to be a super long chapter with four short parts. Of course, each part went beyond short, so now there's going to be eleven instead of ten chapters.  
> Chapter 8 (or Chapter 7 part B, I suppose) will be up on Friday.


	8. Chapter 8

**Summary for the Chapter:**

> As the Sanses work on the machine, the Papyrii- well, one of them, anyway- get to work on spaghetti.

**Notes for the Chapter:**

> Starts out from Underfell Sans's POV; also, it's time to play Spot The Tiki Room Reference.

“welp, here goes nothing.” Sans's counterpart said at last, before flipping a switch on the machine. Its screens and one or two other notice lights flickered to life, casting a pale blue glow across the workshop. The two skeletons waited for a tense moment. Nothing exploded, and nothing turned itself off.

Finally.

“is it doing anything useful?” Sans asked, hunched forward on the sofa, impatient to get the whole ordeal over with.

The other skeleton fiddled with some switches, pressed some buttons, and stared at a screen for a second. Then he nodded and turned to face Sans, leaning against the machine.

“it is now. giving readouts on local space.” he said. “unfortunately, there's absolutely nothing wrong. heh. there's a sentence that's never been said.”

Sans clenched his jaw and narrowed his sockets. “is that it? can you destabilize it?” he asked.

His counterpart heaved a long-suffering sigh. Sans started grinding his teeth again.

“perhaps.” the other skeleton said. “it wasn't really designed for that, though. if you're gonna be twisting the laws of physics the way this thing does, you usually want everything to be as calm as possible when you start. but hey. maybe-”

“then what was the point of bothering with it?” Sans snarled. “all you're ever do with it is see that nothing is changing. what are you gonna do? wait for space to screw itself up ag-”

“actually, yes.” the other Sans said in his usual insufferable manner.

Sans stared at him incredulously, and made to speak again. His counterpart raised a hand to cut him off.

“stop talking while i'm interrupting.” he said. “if my idea about- heh, if my idea was correct, we won't be waiting that long.”

A chill ran down Sans's spine, but he resolutely ignored it, instead scowling daggers at his counterpart, who calmly stared back. This impromptu staring contest was going to go nowhere fast and stay there, Sans realized with some irritation, seeing as neither of them really needed to blink, but he wasn't about to stop and lose face now, so he kept at it.

For a moment, nothing happened. The staring contest, seeing that it wasn't going to be resolved anytime soon, unhappily settled down in nowhere for lack of anything better to do. Then, suddenly, the light from the screens changed, going from pale blue to a tetchy shade of scarlet instantaneously.

Sans jumped off the sofa and his counterpart spun around. Both skeletons stared at the screens, amazed.

“is _that_ it?” Sans asked nervously. His counterpart nodded dumbly, grinning even more widely than usual.

The room still seemed chilled to him, but he could feel sweat starting to roll down his cranium. His legs felt weak, and he stumbled back onto the sofa. Unwittingly, his pupils darted warily about the room, and though he never would have admitted it aloud he could've sworn he felt thousands of unseen eyes gazing back at him from the shadows, which in his peripheral vision took on the shapes of faceless threats and an angry, terrifying figure towering even taller than his brother.

His counterpart gave him a look equal parts concerned and disturbed. What was he so sad about? Had his… Sans clenched his jaw.

“what're you looking at?!” Sans burst out angrily, before the other skeleton could open his stupid mouth to say anything.

He closed his orbitals for a moment and exhaled sharply. “can i crossover now?” he asked, getting back to his feet.

The other skeleton looked taken aback. “why you? uh, in case you're forgetting, i'm the one who's not supposed to be here.” he said, raising a brow ridge.

“yeah, but it's my brother who's lost over on your side, you dastard. who knows where he is now, no thanks to you, and i'm gonna be the one to bring him back!” Sans snarled, seeing red.

The other Sans opened his mouth to protest, but Sans cut him off. “i don't trust you.” he said menacingly.

His counterpart rolled his eyes, and his sockets narrowed in frustration. He sighed, and then looked Sans straight in the eye. “fair enough. but think about this a minute. if i crossover-”

“D o n ' t   e v e n   t h i n k   a b o u t   i t.”

Sans ducked into the lower levels of space and felt it burst outwards around him. Nonetheless, he pushed onwards with the turbulence, making for the main room of the house- if his counterpart had done this and survived, he could, too.

 

His evil twin's pensive mood had not escaped Papyrus's notice. The black-and-red skeleton's unhappy rage had faded into unhappy… something else, and he had done little more than stare longingly out the window since they'd arrived. Did he want the spaghetti that much? Something told Papyrus it probably was more than just the spaghetti. The fact that he was stranded in another universe, and neither skeleton knew how or why or what to do to fix it, probably had something to do with it, but… hmm. Was there a Snowdin in his evil twin's world? Did he miss it? Hopefully the spaghetti would help.

Papyrus knew he sure needed some spaghetti after the morning's events. He preferred puzzles to pitched battles, generally speaking. That, and he still didn't know where Sans was. Probably just slacking off.

A terrible thought struck him. He tried to bat it away, telling the thought that no, Sans was just dodging work as usual, but it was persistent. _What if_ , it said, _Sans is in your evil twin's world?_

If it was true- which, he patiently told himself, it was most certainly not- then that would be a truly deplorable thing. What if all of the other people over in evil twin-land were, well, evil? If Papyrus had a throat, he would have gulped nervously. But he hadn't, so he didn't.

He managed to focus on preparing the pasta, and it was coming along pretty nicely- nothing had caught fire!- when he heard a strange scratching noise behind him.

Papyrus looked out of the kitchen.

His evil twin was no longer standing by the window, but someone else was. The newcomer was a short skeleton, also wearing red and black, and he looked just as grumpy as Papyrus's evil twin had earlier.

“HELLO!” Papyrus said brightly. “ARE YOU SANS'S EVIL TWIN???”

“what.” the short skeleton said blankly. Then, he shook his head. “sure. whatever. is… your 'evil twin' here?”

“PROBABLY..?” Papyrus replied.

Sans's counterpart's sockets narrowed. “whaddya mean, 'probably'?” he growled.

“WELL, HE WAS STANDING RIGHT THERE A FEW MOMENTS AGO.” Papyrus explained helpfully. “WHERE YOU ARE NOW.”

The other skeleton's orbitals went pitch dark. 

**Notes for the Chapter:**

> Sorry for the delay.  
> Underfell Sans didn't think that one through, did he?  
> Chapter 9, which is a tad short, will be up on Monday.
> 
> Also, shout-out to saccharinepeccadillo.tumblr.com, for drawing an awesome picture of Underfell Sans inspired by this fic!


	9. Chapter 9

**Summary for the Chapter:**

> In the Undertale world, Papyrus realizes something's wrong and Underfell Sans realizes he really should have thought that through.

**Notes for the Chapter:**

> Starts out from Undertale Papyrus's POV.

“what?! no. no, no, no!!! that- no! the odds, that… augh!!!” Sans's evil twin spluttered angrily. He ran a hand along his cranium, and turned his back to Papyrus. “this wasn't supposed to happen…”

“THE ODDS OF WHAT? WHAT HAPPENED?” Papyrus asked, with more than a little concern. Why was his brother's evil twin so upset? And what did his sudden appearance have to do with Papyrus's evil twin's disappearance? He really had to think of a better way of referring to these strange skeletons, Papyrus mused.

“SAY, IF YOU'RE SANS'S EVIL _TWIN_ , AND SINCE SANS IS MY BROTHER, DOES THAT MEAN WE'RE ALSO RELATED???” Papyrus wondered aloud, hoping to get an answer this time.

Not-Sans turned, and gave Papyrus another incredulous look. Perhaps that was something that could be discussed later, Papyrus decided. Preferably with a plate of spaghetti.

“DO YOU KNOW WHERE YOUR BROTHER WENT? HE WAS GOING TO HAVE SOME SPAGHETTI...” he said, trying to get to the bottom of the situation. Hopefully it wouldn't upset not-Sans too much.

The shorter skeleton glowered at him. For a brief, silent moment, Papyrus thought his right eye socket looked a bit… reddish, but he chalked it up to a trick of the light.

Then, not-Sans's expression changed. His sockets narrowed slightly, and he cocked his head, looking at Papyrus analytically. Papyrus looked at him nervously; this Sans was disturbing, both because he was so different from his brother, and because of how uncannily similar he was.

“he's not coming back here. he's back in our world.” the sharp-toothed skeleton said at last. His rictus grin widened, and he gave an equally mirthless laugh, holding his head in his hand.

“W-WELL, THAT'S A GOOD THING, RIGHT?” Papyrus said nervously. Not-Sans's behavior was worrying him. “HE WANTED TO GO BACK.”

Not-Sans rounded on Papyrus, fists clenched. “oh, will you just shut up and let me think?” he snapped angrily, baring his wickedly pointed teeth. “stupid loudmouth…”

The rude outburst took Papyrus completely by surprise. But before he could protest the insult, not-Sans's demeanor entirely changed. He leaned back and drew his hands closer to him, while his expression changed from frustration to one of horrified realization. His orbitals widened, and he quickly sucked in a breath.

Papyrus cast a puzzled glance over his shoulder to see what not-Sans was looking at, but saw nothing behind him. He looked back at the shorter skeleton, expecting him to have vanished the way his own brother usually did, but he was still there, and still looked frightened. With an unsettling shock, Papyrus realized that not-Sans was scared of _him_.

“YOU DON'T HAVE TO BE SCARED OF ME!” Papyrus said unhappily, almost pleadingly, hoping to assuage the other skeleton's fears. “THAT WASN'T VERY NICE, BUT- I'M NOT GOING TO DO ANYTHING TO YOU!!!”

Gradually, some of the defensiveness seeped out of not-Sans's bearing. He kept a wary eye on Papyrus, however, and said nothing. _What was he expecting?_ , Papyrus wondered. His evil twin hadn't… well, he really hadn't seemed all _that_ evil. Wrathful and blustering, true, but not… _that_. More of a villain-wannabe than the genuine article.

Papyrus decided that the other world was not at all to his liking.

 

After a split-second, Papyrus's doppelgänger gave an exclamation- something about spaghetti- and rushed back to the kitchen. Sans was glad to see the back of him. He'd really made an idiot of himself this time, what with his monumental mistake of blindly crossing over without coming up with a way to make sure it wasn't a one-way trip. And then he'd gone and provoked a stranger whose power he didn't know and whose reactions he couldn't predict- and let his fear show through to boot. He was lucky that the monsters of this Underground were all soft.

“stupid...” he muttered at himself, smacking a palm to his forehead.

Now that he was alone, Sans ducked out of normal space. Around him, he could feel space reverberating as it had after the first crossover, but not the explosive shockwave he'd felt when he'd gone from his world to this one. He went back to the house, and saw the green sofa and- why was the paperweight covered in sprinkles?

He was stuck here, he realized. A cold, creeping dread washed over him. Well, at least Papyrus was back in their own world. Sans had managed that much.

Sans wondered if there had ever been a chance of getting them both back. If he had managed to cross over without displacing Papyrus, they still wouldn't have been able to return, because space wouldn't have been properly unstable or some such nonsense. And if he'd allowed his counterpart to cross over instead, Papyrus still would have been stranded for the same reason; it was only by pure dumb luck, good or bad he could not tell, that Sans had happened to take a shortcut to the exact point that Papyrus had been standing. No matter which way he looked at it, someone got left stranded.

From the kitchen, Papyrus's counterpart shouted something about pasta. Sans just ignored him. The only shot he had at going back to the world he knew relied on him staying exactly where he was. It was a pretty long shot, but he had left nothing to lose in trying.

He put his hands back in his pockets, and he could feel his hand colliding with his phone. Sans froze. _His phone_. Maybe this wouldn't be such a long shot after all.

He quickly pulled it out of his pocket and began dialing his own number. Amazingly, he could hear it ringing. Space wasn't unstable enough to crossover anymore, but apparently the divide between the two universes was weakened enough to let a cellphone signal go through.

After a few moments, in which Sans's heart would have been pounding hard if he had one, the call was picked up.

“my fridge is running fine, thanks for asking.” his counterpart's voice said from the other end.

**Notes for the Chapter:**

> So yeah, this one wound up a little shorter.  
> Felsans tends to freak out if it looks like someone's going to be angry with him. Most of the time he'd freak out inwardly or try to intimidate the other monster into leaving him alone, but what with everything that's happened lately he's not entirely on the ball at the moment.
> 
> I'll post Chapter 10 on Thursday.


	10. Chapter 10

**Summary for the Chapter:**

> Sans is still stuck in the world of Underfell with Papyrus's counterpart, but a solution may be in sight.

**Notes for the Chapter:**

> I headcanon Dadster.

It happened swiftly and without warning, like a bolt from the blue- not that Papyrus knew what those were. For a split second, he felt as if he was being both shoved and pulled, not in any particular directions, but from all directions at once; left and right, up and down, in and out. The room around him blurred away into something utterly incomprehensible, and it seemed that space itself- not any matter within space, but the fabric that held everything together- was shaking wildly. Suddenly everything stilled and came back into focus, and Papyrus found himself not looking out of the window in his counterpart's home, but at the dark pane of a window in his own house. That, at least, was a relief, although he felt a smidgen of disappointment; he'd actually been starting to look forward to that spaghetti.

He turned and looked around. All of the lights were off, just as he had left it, and as far as he could see, he was the only one in the house.

But then again, Sans sometimes didn't bother to turn the lights back on when he was there.

“SANS? ARE YOU THERE?” he said in an authoritative manner.

“uh. in a manner of speaking.” Sans's voice answered from behind him, in a much calmer voice than usual.

Foreboding shot up Papyrus's spine like lightning. Slowly, he turned back and looked down. A short skeleton was standing there, in a comfortable-looking blue hoodie, and from the different clothes and wildly different demeanor it was apparent that this was Sans's counterpart.

“hey.” he said lazily.

The other monster's laid-back attitude grated on Papyrus's nerves like nails on a chalkboard grate on human ears. Papyrus still didn't understand a single thing that had happened that entire incomprehensible, risible day, which was infuriating enough on its own, but to see that someone else- someone who he knew was weaker, inferior- was going through the exact same thing and yet was still able to act this… _unbothered_ was what pushed him over the edge.

“WHAT ARE YOU DOING HERE?” Papyrus boomed. He loomed over the smaller skeleton, making sure he was aware of the extent of Papyrus's rage. “WHAT'S GOING ON? TELL ME!”

“that's… a bit of a long story. and don't pa _tell_ a me what to do.” not-Sans said.

Papyrus was speechless. Had he really just–?

“do you want to step outside?” the other skeleton continued (to Papyrus's dismay).

Not-Sans was still grinning that insufferable grin. He couldn't mean what Papyrus thought he meant. Could he? Was he really so stupid as to challenge _him_ , the Great Papyrus, to a fight in such a manner?

“'cuz i really think you need to _chill_.” Sans's counterpart said, winking his left eye. Somehow, his smile had become even wider.

Papyrus's frown deepened out of anger and utter bewilderment. A laid-back Sans that cracked lame jokes at inopportune times? It felt like an unwanted flashback to his childhood. Of course, his own counterpart had been extremely childish, too; at least, Papyrus could remember acting like that as a child and hadn't known better. Was that what the other world was? Some alternate reality where no one had grown up mentally?

After the terrible day he'd been having, Papyrus did not want to sit back and let an obnoxious parody of his brother ruin it even further with awful jokes and unpleasant reminders.

“SHUT UP!” he roared. “AND GET OUT OF HERE!”

 

Welp. Papyrus's counterpart was indeed thuggish, perpetually angry, sharp-toothed, and had a thirteen-year-old's fashion sense; besides that, he also had several long scars criss-crossing over his left eye socket. Meeting him was just as unpleasant as Sans had suspected. Though he kept a smile pasted on his face, he was horrified; not by what might happen to him, but by whatever it was that had turned his brother- the friendliest and sweetest monster he knew- into _this_.

“heh. a.) no.” he said. “and b.) what do you think i'm trying to do?”

Somehow, his bull-headed counterpart had actually succeeded. Partially. Sans felt a twinge of sympathy for the other skeleton. He could just imagine how frustrating it was, being trapped in another world because _someone_ didn't think things through before recklessly crossing over. Now, why was it so easy to imagine it…? Oh, right. Because he was feeling the exact same frustration at that very moment. If the other him had put an ounce of thought into it, or let Sans finish his proposition- which would have been to use the machine back home to send Papyrus's counterpart back here- they wouldn't be in this mess.

Well, the situation wasn't entirely bleak. He knew the exact point to crossover if space destabilized again… _if_ Papyrus's counterpart was still standing where he had been when he'd been displaced, and _if_ his counterpart had used his head and was staying put. Okay, so that was a lot of ifs.

Sans sighed inwardly. He was going to need to take _so_ many breaks after this was all over.

“hey. could you, uh, stand somewhere else? i need to be in that exact spot.” he said, deadpan.

Not-Papyrus looked bewildered and, for some reason, affronted. “WHAT.” he said, which to be fair was a perfectly natural response to Sans's statement.

“IS THIS SOME SORT OF IMBECILIC PRACTICAL JOKE?” he demanded. The lean skeleton raised a fist and shook it at Sans. “IF IT IS, I'LL-”

“nope. this is absolutely serious. so. uh. move over.” Sans said hurriedly, before not-Papyrus could continue. He knew with whatever it was he had been going to say next, they were not words Sans wanted to hear being said in his brother's voice.

Papyrus's counterpart, however, remained exactly where he was, and kept scowling at Sans. Was he capable of making _any_ other expressions? Did he remember how to smile, for instance?

“heh. budge. i mean it.” Sans's eyes went dark, but not-Papyrus didn't show the slightest hint of concern at this.

Sans closed his eyes for a moment. “fine.”

Without any further ado, he strode up to Papyrus's counterpart, turned him blue, and shoved him out of the way, before turning his back on him.

He pulled his phone from his pocket, and checked his voicemail. The strange call from earlier was listed there, and the caller ID showed not a number, but a string of random, distorted shapes that continuously changed.

Sans called whoever it was, and while he waited to see if anything would pick up, he heard only silence coming from the phone. Behind him, he could hear that Papyrus's counterpart had overcome his surprise, and was now stomping up to him; without turning around, Sans turned him blue and magically pushed him across the room.

Then, harsh, wrathful roars and agonized shrieks erupted from Sans's phone. He held out the phone away from his skull for a moment, and then slowly drew it back, close enough to talk into but far enough for the distorted, unrelenting noises not to blow out his ossicles.

“WHAT IS THAT?!” not-Papyrus asked, shocked.

“hello.” Sans said into the phone. “i don't know if this is your fault, or if you're even who i think you are, but whatever it is you've been doing could you, uh, maybe do it a third time? seeing as we screwed up last time…”

The sounds changed instantly, to a booming, mocking cackle dripping with malevolence. Sans was beginning to understand why his counterpart had acted so terrified earlier.

Before Sans could say anything else, another noise joined in the general cacophony. It was just as freakish and unnatural, but it was not mocking or malicious; it sounded like someone was trying to say something, but was as incomprehensible as people talking from far away. As the second “voice” went on, the harsh cackling stopped abruptly, and Sans thought he heard the first voice growl in frustration. Then, both voices cut out all together, and the only sound to be heard on the other end was a deep shaking.

Sans hung up, and as soon as he did so his phone started ringing. The caller ID showed that he was apparently trying to call himself.

“my fridge is running fine, thanks for asking.” he said casually. “guess you've learned a thing or two today about thinking things through, huh?”

“shut up.” his counterpart growled. “can you d-destabilize space? c-can i go back?”

“hopefully. you haven't gone anywhere, have you?” Sans asked.

“of course not!” not-Sans shouted angrily. “i'm not an idiot!”

“IS THAT MY BROTHER???” Papyrus's counterpart asked, with clear surprise that he quickly changed into domineering anger. “WHERE IS HE?!”

“hang on a sec.” Sans said over his shoulder, before returning his attention to the call. “right. just keep doing nothing and hopefully this'll all be sorted in a minute. won't see ya.”

He heaved a sigh. “finally.”

**Notes for the Chapter:**

> A very merry Christmas tomorrow to all who celebrate it, and a great day anyway to those who don't!
> 
> When Felpyrus calls UT!Sans "inferior", it's not due to the fact that he's Sans, it's mostly because he comes from the Undertale world, which Felpyrus believes is full of people who are soft and weak and, therefore, inferior.  
> Ossicles are the bones in one's ears.
> 
> As a side-note, I've gone back to previous chapters and made a few revisions here and there, mostly to make sure characterization is consistent, and the implications of the story match my intentions for the characters.
> 
> Chapter 11 is the end and epilogue of the story, and I'll be posting that on Sundayl


	11. Chapter 11

**Summary for the Chapter:**

> The Sans-es swap places, and the story reaches its epilogue.

Sans took a shortcut, and as soon as he felt the disturbance in space he went right back to the point where he had been. He emerged in a better-lit room with a more homey feel to it. His counterpart was nowhere in sight- but Papyrus was.

“SANS! YOU'RE HERE!” he exclaimed happily, as he rushed over and picked Sans up in a hug.

“yep. you good, bro?” Sans asked. Papyrus looked like he was okay, but if he'd been hanging around his counterpart for the day…

“I AM FINE. BUT ARE YOU ALRIGHT? YOU'RE NOT HURT, ARE YOU?! I CAN ONLY BEGIN TO CONTEMPLATE WHAT THE EVIL TWIN WORLD MUST BE LIKE…”

Papyrus trailed off, eyes wide.

“i've had better times, but i'm okay.” Sans replied in a laid-back tone. Nonetheless, Papyrus walked over to the sofa and deposited Sans on it before sitting down next to him. “so, 'evil twin'?”

“CAN YOU THINK OF ANYTHING BETTER TO CALL THEM?” Papyrus asked defensively. “EVIL ME, EVIL YOU- THEY EVEN HAVE AN EVIL ROYAL GUARD IN THEIR WORLD! THEY SEEMED FAIRLY MISERABLE, THOUGH. WHAT HAPPENED? HOW DID YOU GET STUCK THERE IN THE FIRST PLACE? HOW'D YOU GET BACK???” the taller skeleton asked.

Sans sat back and closed his eyes, then shrugged. “dunno, bro.”

“WELL, THANK GOODNESS YOU DID! ARE YOU SURE YOU'RE ALL RIGHT? LOOK, I'M SURE NO ONE WOULD MIND IF YOU TOOK THE DAY OFF, CONSIDERING THE CIRCUMSTANCES…”

“thanks, bro.” the shorter skeleton said, touched. He had been planning on doing that anyway, but still appreciated the gesture. “yeah, the other world wasn't all that friendly. i've been feeling pretty bone-ly today…”

Papyrus looked extremely put out. “REALLY, SANS?!” He heaved an exaggerated sigh. “DON'T MAKE ME RE-THINK THE SUGGESTION…” he said with false severity.

After a moment, he brightened again.

“WOULD YOU LIKE SOME SPAGHETTI?” he asked, getting up from the sofa. Then he sighed, and a sad expression flitted across his face. “MY EVIL TWIN NEVER GOT TO HAVE ANY… OH WELL. NOW THERE'S MORE LEFT FOR US! NYEH-HEH-EH!”

After the day's crazy shenanigans, even the prospect of Papyrus's spaghetti was uplifting.

“sounds great.” Sans called, as the taller skeleton strode back to the kitchen.

At about that point, Sans started dozing off. Papyrus came back a few minutes later with two plates of spaghetti, and plopped down on the sofa beside him. Although he was half-asleep, the shorter skeleton was aware of his brother setting a plate on Sans's head and turning on the newest episode of Mettaton's show. After seeing it in so many timelines the memories of it all compounded, so Sans practically had it memorized, but for once he didn't feel sick of it.

Who knew how this timeline was going to turn out? Sans didn't care, at least, not for the moment. Right now, things were going well. Tomorrow was another day.

 

Being displaced was a lot more unpleasant than doing the displacing, Sans mused as his own house came into focus around him. With any luck, he'd never have to do either again, and this frighteningly unfamiliar scenario was coming to an end at last.

“SANS!” Papyrus shouted from across the room- what was he doing over there?- before clomping over. “THIS HAS BEEN ONE OF THE WORST DAYS EVER!!” he said vehemently. “WHAT HAPPENED?!”

The taller skeleton glared down at Sans, who adopted an indignant expression. “what are you asking me for, you clod? i didn't like it any more than you! it wasn't my fault!” he complained.

“HRMPH.” Papyrus crossed his arms dramatically and cast an exaggeratedly-angry glance upwards, as people seeking patience are wont to do; or perhaps he was practicing his intimidating glare on the ceiling again. “'YOU DIDN'T LIKE IT'? THAT BARELY QUALIFIES AS AN EXCUSE, YOU SHIFTLESS REPROBATE.”

“pfft. goon.”

“MALINGERING IMP.”

“lowlife.”

“INDOLENT CUR.”

Sans still felt somewhat anxious, even though the crossing over was, hopefully, over. Papyrus's day really must have gotten to him; he wasn't in his top insulting form at the moment. He'd used three synonyms for “lazy” in a row. Usually, he tended to mix things up with his scorn, whenever they had longer exchanges of slander, going after Sans's work ethic, intelligence, and cowardice all at once, not one after the other. Sans usually just stuck with one-word insults, himself. They were easier to come up with on the spot, and a lot quicker to sling at other people.

“oaf.”

“CRAVEN SKULKER!”

Well, at least Papyrus had gotten off one shot at Sans's fearfulness before they lapsed into an awkward silence. Still, it was clear that something had shaken Papyrus badly. He was looking upwards again, and Sans glanced down. He wanted to ask about it, but he knew that Papyrus would never admit to feeling afraid or sad no matter how much he did. Besides, he had a guess. Seeing their counterpart's home had gotten to him, too.

It reminded him so much of how things used to be, and for that reason, he wanted to shove it out of his mind as soon as possible- dwelling on the past never helped anyone in the Underground. Papyrus was probably trying to do the same thing.

“WELL. WE ACCOMPLISHED NOTHING TODAY.” the taller skeleton said at last.

Sans shrugged. “eh. we survived.” he suggested. Perhaps this time, Papyrus would just accept the idea and leave it at that.

“THERE IS THAT;” Papyrus granted; Sans waited with bated breath. “WE CAN ACCOMPLISH SOMETHING TOMORROW.” Papyrus fixed Sans with a Look that loudly said, 'YOU'D-BETTER-WORK-TOMORROW-OR-SO-HELP-ME…'.

The shorter skeleton gritted his teeth. No, Papyrus would never just agree with him outright.

If Sans was going to get a much-needed break or nap out of this mess, he was going to have to be really sneaky about it. He knew that if he so much as took an extra long blink in his brother's presence the next day, Papyrus would be on his case instantly. This timeline was going to be even rougher than usual, the way things were going.

“y-yes sir.” Sans said nervously. “i won't let you down.”

“GOOD.” Papyrus said with a quick nod. “SEE WHAT YOU CAN DO WHAT YOU CAN FOR THE REST OF THE DAY. PROBABLY NOTHING.” he added with a frustrated sigh. “BUT TOMORROW, WE'RE BACK TO WORK AS USUAL.”

He turned and stalked off toward the stairs. “IF NOTHING ELSE, AT LEAST GO CHECK ON YOUR TRAPS!” he said over his shoulder.

Sans watched him go back to his room, and then sat down on one of the chairs with a sigh.

Tomorrow was yet another day.

**Notes for the Chapter:**

> The last chapter... a truly bittersweet update. On the one hand, the story is completed and published in its entirety, which is a pretty good feeling. On the other hand... drat, I finished.  
> Also, confession time: I hate sad endings. Good Lord, I despise them. The only reason I ever put up with any angst is if I'm holding out for a happy ending. But I feel like I ended the Underfell bit on a sad note. So how could I have written this in good conscience? Well, for one thing, this felt like the most logical/sensical conclusion for the Underfell Skelebros.
> 
> But also, I'm planning to post a sequel.  
> When I first got the idea for this crossover, I thought I would extend it into a Post-Pacifist/Post-Redemption setting (so I could have both Undertale bros go to Underfell without breaking my "no hurting Papyrus" rule). So I'm going to. First, though, to make sure I don't clutter up the narrative/dialogue of the second crossover with clunky exposition, I'm going to write some chapters charting the character development of the Underfell bros in a Redemption route, and also probably of a few other characters, too. I'm not sure when I might start uploading sequel stuff. No sooner than Sunday for certain. The character development one will be called A Path to Redemption.
> 
> Thank you all SO MUCH for reading this! It's been surreal and amazing. I never dreamed my fic would get this much attention!
> 
> Six thousand hits...? SIX THOUSAND hits?! Holy frick, thank you all so much!!!


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